Omniduct

This is a relatively small release, but which has slightly different default behaviour, making it a tick over to the next major release.

Improvements:
- Authorisation is now actually passed through to backends in Presto, Hive, Druid and Neo4j clients when `username` is not None. To avoid default usernames being passed, use `username=False` in the constructor.
- Allow customisation of the `source` name for Presto clients.

Bugfixes:
- Fixed `requests` being loaded at import time, despite supposedly being a runtime import.
- `None` results from presto polling are now correctly handled.

Features and Enhancements:
- Added warning when users push dataframes using `HiveServer2Client.push` when default table properties are implicitly used.

Bugfixes:
- Fix reference to `self.remote` in WebHdfsClient constructor, which causes the client to be prepared at creation time rather than lazily deferred until first use.
- Fix empty StringIO buffer being returned when opening a non-existent file with mode 'r'.

Features and enhancements:
- Allow all `host` specifications provided to a `Duct` instance to include the port. This port will take precedence over any value supplied via `port`.
- Supplying a list of hostnames to a `Duct` instance as the `host` value will result in naive load-balancing (i.e. the hostnames will be randomly sorted and the first accessible hostname/port will be chosen).
- Added support for automatically configuring `WebHdfsClient` instances via a cloudera XML configuration file.

Bugfixes:
- Fix typo/bug in `LocalFsClient.open()` due to missing `self` reference in function definition.

This release marks the graduation of omniduct from a fledgeling project to a publicly supported body of code. Core functionality is now fully documented. Refer to documentation for a full rundown of functionality.